The current general workforce faces numerous obstacles in conducting daily work activities. Increased population often means greater traffic on the nation's highways resulting in longer commutes and greater pollution. Also, economic conditions often require both parents of many families to work in order to make ends meet. This means parents are not at home to take care of children. This problem is even greater in single parent homes. When a child gets sick, parents must take a day off from work to tend to the child, losing both valuable business and vacation time.
Fortunately, current technology is providing options for employers and the workforce to accommodate these realities. One such solution is to move the workplace into the home. Modern communications technology is aimed, in part, at facilitating this move. A specific instance involves the use of simultaneous voice and data (SVD) modems to allow the operation of a personal computer and a telephone at home in communication with similar equipment in the workplace. In this case, voice and data signals are modulated together by an SVD modem at a remote site and sent over telephone lines to a second SVD modem at the workplace. The second SVD modem will demodulate the voice and data signal for use at the workplace. In particular, the voice signal may be sent to another extension at the workplace to communicate with other workers or to another telephone outside of the workplace to communicate with customers or other interested parties.
It is at this point that an old problem inherent in the structure of the telecommunications system can degrade the quality of the voice signal transmitted in this manner. Many telephones use two wires with which to send and receive voice signals. Typically, the public-switching telephone network will route these voice signals over two wire pairs. Four wire transmission uses a two wire pair to receive a voice signal, and another two wire pair to send a signal. Consequently, at some point, the signal must be converted from four wire to two wire and vice versa. This is commonly achieved with the use of a hybrid transformer.
A problem that accompanies the use of such transformers is the creation of so called "talker echo" due to an impedance mismatch in the transformers. When a voice signal is sent by a telephone transmitter, the hybrid transformer will reflect the same signal back to the same telephone where it is heard in the receiver. The signal that is heard in the receiver will be delayed depending on how long the signal takes to complete the round trip. Consequently, the talker will hear their own voice as they speak, only delayed by a finite period of time, i.e. they will hear an "echo" of their voice.
Modern telecommunications companies have essentially solved this problem by the introduction of echo cancelers. Echo cancelers essentially subtract a copy of the original signal from the return path so that the echo signal is effectively "canceled out." The echo cancelers are typically placed in the long distance components of the public-switching telephone network ("PSTN"). These cancelers typically activate where echo signals occur with a delay of greater than thirty-five milliseconds.
The use of SVD modems in the remote office configuration as discussed previously creates a new problem relative to echo cancellation. Generally, SVD modems use quadrature amplitude modulation techniques to send data at relatively high baud rates which are desirable in remote office applications. SVD modems transmit both voice and computer data information simultaneously by either analog simultaneous voice and data transmission (ASVD) or digital simultaneous voice and data transmission (DSVD). ASVD modems combine voice and data signals by generally imposing the voice signal on top of the data signal, for example, according to V.61 voice series communication protocol. DSVD modems intersperse a digitally coded representation of the voice into the data stream, for example, according to V.70 protocol for time multiplexing, thereby creating a combined signal to be transmitted across telephone lines. The essential operation of ASVD and DSVD modems as such is an expedient known to those skilled in the art.
The time necessary for the electronic process of modulating or voice coding a combined ASVD or DSVD signal using either of these techniques delays the voice transmission several milliseconds. A similar delay is created when the transmitted signal is received and demodulated by the second SVD modem. These small delays are added together, resulting in an overall delay.
In the remote office or similar environment, hybrid transformers in the network between the second SVD modem and the receiver of the remote office call may reflect an echo signal back to the second SVD modem. If there is no echo cancellation in the PSTN between the receiver and the second SVD modem to cancel this echo signal, as is often the case for local telephone calls, then a delayed echo will be heard by the individual in the remote office. This echo signal can be confusing, tiresome and disruptive, affecting the productivity of employees.
Because of the importance of remote office technology and as a result of the inadvertent creation of disruptive echo signals in the use of SVD modems in the remote office or similar applications, there is a need for an SVD modem with echo canceling capability to be used in a remote office or similar application to eliminate the harmful echo signal.